The present invention relates to a portable radio apparatus, and in particular, to a portable radio apparatus capable of providing an impedance match by use of an antenna and a matching circuit having a simple configuration for all frequency components in a frequency band to be used.
Description of the Prior Art
In a personal digital cellular (PDC) system as a Japanese standard system and in a global system for mobile communications (GSM) as a standard system in Europe, time division multiple access (TDMA), in which each station sends or receives signals only using a particular time slot allocated thereto, is employed to improve efficiency of frequency utilization.
In the PDC system, different frequency bands are assigned respectively to an up communication and a down communication to thereby multiplex communications. A frequency band ranging from 810 MHz to 885 MHz is allocated to the down communication, and a frequency band ranging from 885 MHz to 960 MHz is allocated to the up communication. In the GSM system, a frequency band of 935 MHz to 960 MHz is allocated to the down communication, while a frequency band of 890 MHz to 915 MHz is allocated to the up communication.
With increase in the capacity of contents communicated via wireless communication in the future, it will be required to increase the transmission speed. It is hence considered to be necessary depending on cases that a duplex function is installed to increase the transmission speed using all receiving slots in data communication.
In the duplex communication, transmission and reception are required to be carried out at the same time. Therefore, it is necessary for a portable radio apparatus having the duplex function to include an antenna covering a frequency band for up communication and a frequency band for down communication.
It has been well known that a portable radio apparatus today such as a portable telephone, a radio apparatus including a communicating function, or a personal digital assistant (PDA) including a communicating function has an antenna characteristic which is remarkably influenced by the length of a casing or a cabinet thereof.
FIG. 1 shows a relationship between the cabinet length of a portable radio apparatus and the antenna characteristic thereof. In the antenna characteristic, a bandwidth is plotted while the cabinet length is changed such that the return loss is equal to or less than −10 dB when an optimal matching circuit is used. The ordinate indicates a fractional bandwidth (%) of the antenna, and the abscissa denotes a ratio (L/λ) between the cabinet length (L) and wavelength (λ). In this example, it is assumed for calculation that the cabinet length is equal to about 0.14λ and the antenna is a helical antenna with a height of about 0.045λ.
In the present market, a holding type portable telephone (which can be folded at a hinge section) and a portable telephone the cabinet length of which can be elongated by rotating or drawing out two cabinet sections are in widespread use. In such a telephone, the cabinet length in the elongated state is about 0.2 m in general. The length corresponds to 0.54λ to 0.6 λ in the 800 MHz band. As can be seen from FIG. 1, the bandwidth of the antenna is about 5.6% in this situation.
In the 800 MHz band communication, the PDC system uses about 11% of the band, while the GSM system employs about 8% to 9% of that. Therefore, regardless of whether the PDC or GSM system is adopted, the existing portable telephone having an antenna bandwidth of about a little under 6% cannot secure a sufficient band. In other words, the antenna element of the telephone effectively functions only for part of frequency components of the frequency band used in the PDC or GSM system.
As just described, when the duplex function is added to the conventional portable radio apparatus, there arises a problem of the narrow bandwidth of the antenna.
The portable radio device of the prior art cannot enlarge the antenna bandwidth because the matching circuit thereof can only match with the antenna in a narrow frequency band. That is, a matching circuit for transmission matches with the antenna only in a frequency band for the up communication, and a matching circuit for reception matches with the antenna only in a frequency band for the down communication.
Therefore, the conventional portable radio apparatus includes matching circuits respectively optimal for different frequency bands, and implements the duplex function by selecting therefrom a matching circuit appropriate for a frequency band to be used. FIG. 2 shows a configuration of a radio apparatus of this type. The apparatus includes a parallel resonance circuit for duplex transmission 5, a TDMA transmission circuit 19, and a TDMA reception circuit 20. These circuits are connected respectively via switches 3, 16, and 17 to respective points between a matching circuit 2 and a radio section change-over switch 7.
The circuit 5 is used at packet transmission and is hence hardly used on the side of the human head. Therefore, the circuit 5 has constants adjusted so as to simultaneously cover the transmission and reception bands in a free space.
In a call using voices and sounds, the portable radio apparatus conducts communication using the TDMA system and hence is employed on the human head side. When the duplex transmission circuit 5 is used in the TDMA communication, the matched state becomes worse on the human head side and hence the antenna characteristic is also deteriorated. To overcome this problem, the TDMA transmission circuit 19 is adjusted such that, whether used in the free space or on the human head side, the transmission band only is in an optimal matched state.
The operation of the apparatus will now be described. In communication employing the duplex function, the switch 3 is closed and the other switches are opened. Thereby, only the parallel resonance circuit 5 operates to cover the transmission and reception bands at the same time.
In the TDMA communication, the TDMA transmission circuit 19 and the TDMA reception circuit 20 are operated according to control signals respectively synchronized with transmission timing and reception timing. In this situation, the circuit 5 and a 1.5 MHz circuit 21 are separated from the antenna 1 respectively by the switches 3 and 18. Therefore, in the state of the audio call, a satisfactory characteristic can be attained in both the transmission and reception bands.
In the portable radio apparatus, the duplex function can be implemented in the 800 MHz band. However, the change-over operation is required to select an appropriate matching circuit according to the communication system. This leads to a problem that the circuit configuration becomes complex and the number of parts increases. In association with the increase in the circuit size and complexity of the circuit, the loss in the matching circuit increases and hence the radiation efficiency is deteriorated.
Since the apparatus also requires switches for respective communication systems, the apparatus consumes larger amounts of power. Moreover, a logical circuit to control each of the switches is also increased in size. This results in a problem of a larger mounting area of each matching circuit.
Japanese Patent Application laid open No. 9-307331 entitled “A Matching Circuit and An Antenna Device Using the Same” describes an antenna device for a radio apparatus communicating signals of frequencies in a wide range without conducting a change-over operation between matching circuits.
However, according to the invention disclosed in the above patent application, a resonance frequency of a matching circuit is adjusted with a variable condenser or capacitor and hence there is additionally required a control circuit to change the capacity of the condenser. Therefore, since the circuit size of the matching circuit cannot be reduced, the matching circuit loss becomes larger as in the above-described prior art carrying out a changeover between matching circuits, and hence the radiation efficiency is lowered.
Besides, in portable telephones, a kind of the portable radio apparatuses of today, the call channels become insufficient due to increase in the number of subscribers when only the 800 MHz band is used as before. Therefore, it has been advised that users subscribe in communication services provided with the use of electromagnetic waves in the 1.5 GHz band, 1.9 GHz band, or a higher frequency band.
This has resulted in the development of a portable radio apparatus called “dual band apparatus” so that one portable telephone can use two frequency bands. The dual band apparatus is configured to operate using the 800 MHz band and 1.5 GHz band (nearly double the 800 MHz band) or using the 800 MHz and 1.9 GHz.
The dual band apparatus is required to be configured such that a radio circuit to convert a radio signal into an electric signal can operate with two frequency bands and an antenna and a matching circuit to receive a radio signal function in both frequency bands.
However, the conventional portable radio apparatus cannot cover all frequency components of the 800 MHz band with only one matching circuit as described above. To conduct a dual band operation with 800 MHz and 1.5 GHz, it is required that the conventional apparatus using only the 800 MHz band shown in FIG. 2 further includes a matching circuit (1.5 GHz band matching circuit 21) to communicate information through the 1.5 GHz band, a circuit (second frequency band receiver circuit 14) to receive a radio signal in the 1.5 GHz band, a circuit (second frequency band transmitter circuit 15) to send a signal in the 1.5 GHz band, and switches (switches 7c, 11, and 18) to conduct a change-over operation between them as shown in FIG. 3. In the configuration, a control circuit 6 is required to appropriately carry out a changeover in the switches according to a communication system to be used.
In this case, the circuit size becomes greater as compared with the portable radio apparatus using only the 800 MHz band, and hence the loss by the matching circuit and the deterioration in the radiation characteristic appear more strongly.